Mobile concrete mixers having a mixer drum mounted on a truck chassis are well-known in the building industry. Such concrete mixers are used to transport liquid concrete (comprising cement, sand and water) from a plant to a building site where the concrete is to be used. The concrete mixer drum is typically mounted at an angle on the truck chassis with the opening of the drum some height above the ground. The drum is rotated in the first direction to draw materials into the drum during transportation to the building site and it includes internal screws or ribs which expel concrete from the drum through the discharge chute when the drum is rotated in the opposite direction.
Access to the interior of the drum is required for cleaning and maintenance purposes. Accordingly, the drums are typically manufactured including a manhole which is closed by a manhole cover that is securely bolted to the drum using, typically, twelve separate bolts. To gain access to the drum interior, an operator must release each of these bolts and remove the manhole cover, before climbing to the manhole. This is a time-consuming process and the weight of the manhole cover is such that it can be a cumbersome operation to remove the manhole cover out of the way of the access into the drum, particularly if the user still holding the tools.